Cache the source of a test file to avoid reading it twice

[SimenB]

Do you want to request a feature or report a bug?
Feature, I suppose

What is the current behavior?
As of the behavior introduced in #2859, all test files are read from disk twice; once inrunTest.js in order to check for docblocks, and once in jest-runtime to actually transform and run it. This is unnecessary, the content should only be read once.

If the current behavior is a bug, please provide the steps to reproduce and either a repl.it demo through https://repl.it/languages/jest or a minimal repository on GitHub that we can yarn install and yarn test.
N/A

What is the expected behavior?
That the source code is only read once. See @cpojer's suggestion in #2859

I'm a bit worried about the fact that this will end up reading every test file twice and we could potentially change things around by passing a fake FS into ModuleLoader that has a cache, like this:
new ModuleLoader({
…,
cacheFS: {
[path]: source,
},
});
and then later pass that source to transform inside of jest-runtime so that we don't read the same file twice.

The idea was to change jest-runtime to accept a cacheFS property in the constructor which is just a map from path -> source. Then inside of _execModule we could pass on the source that we already read to the transform function somehow. Basically we could do transform(…, {source: this._cacheFS[pathToFile] || null}) – does that make sense?

Please provide your exact Jest configuration and mention your Jest, node, yarn/npm version and operating system.
N/A

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